Current practice in optical sensor calibration is typically performed on a sensor-by-sensor basis, applying a standard procedure to each optical sensor and using the same number of reference fluids for all optical sensors involved. Attempts to reduce the number of reference fluids in a calibration procedure may over-fit the selected data sets and fail to adequately generalize results for downhole application fluids. This is especially problematic for calibration techniques adopting a non-linear mapping algorithm. On the other hand, increasing the number of calibration fluids for each optical sensor has the drawback of low safety and high cost.
Adding to the complexity of current strategies for standardizing optical sensor response is the desire to perform reference measurements of calibration fluids at multiple temperature settings, multiple pressure settings, and different combinations of temperature and pressure settings. Combining measured and simulated optical sensor responses on reference fluids has been an attractive technique to solve the calibration and standardization problem. However, cross-correlation of optical sensor data from multiple reference fluids may be non-linear when no spectral correction is available for each optical sensor. When the cross-correlation is non-linear, it is difficult to develop a method applicable to additional reference fluids based on the measurements of a reduced number of reference fluids. What is missing in optical sensor calibration and standardization techniques is a cross-sensor data linearization method applicable over different optical element designs and different optical sensor configurations in optical tool parameter space.